The EGR (Exhaust Gas Re-circulation) method is known as a technology that is used for reducing problematic NOx (nitrogen oxide) in exhaust gas emitted from a diesel engine.
Moreover, when the EGR method is applied, the amount of fresh air (a fresh intake air flow rate) inhaled by the engine decreases relatively which is prone to cause an O2-deprived (oxygen-deprived) atmosphere in a combustion chamber of the engine when the engine is rapidly accelerated or a fuel admission opening of the engine is rapidly increased.
Specifically, when an EGR valve and an intake air throttle valve of the engine are concurrently used in order to raise EGR rate (a ratio of EGR gas in the air-gas mixture inhaled into the cylinders of the engine, in general), a sufficient EGR rate cannot be achieved only by opening the EGR valve fully. Thus, beside the EGR valve, the intake air throttle valve is activated in a closing direction so as to decrease the inhaled fresh air and enhance the EGR rate (the EGR gas flow rate). As a result, an O2-deprived (oxygen-deprived) atmosphere is prone to occur in the combustion chamber of the engine.
In the case of using the EGR valve and the intake air throttle valve of the engine concurrently, the EGR valve control and the throttle valve control are conventionally performed independently, namely, in an uncoupled mode. For instance, an example technology is disclosed in the patent reference 1 (JP2006-90204), whereby the EGR valve and the intake air throttle valve are controlled by different command signals independently. Thus, in the technology by the reference 1, the number as to the degrees of freedom in connection with valve control operations becomes large. Therefore, the calibration man-hours for determining an optimal control conditions have to be increased.
In order to simplify the calibration work, it is known that the EGR valve movement is associated with the throttle valve movement as if the two valves were integrated into one valve; that is, as shown in FIGS. 11 and 12, a set of the EGR valve opening command and the throttle valve command is forwarded in response to a simple control command signal.
FIG. 12 (as to a known technology) shows an example of the above-mentioned simple control command signal that corresponds to a signal θ in the figure; whereby, the signal θ is a control command signal in the control block diagram (FIG. 12 itself) that is a part of a feedback system in which a signal of the target intake air flow rate, together with a signal of the actual intake air flow rate, produces the control command signal. To be more precise, a target air flow rate calculating means 01 calculates a target air flow rate in response to an engine speed and a fuel injection quantity per shot; the target air flow rate is compared with an actual air flow rate that is detected by an air flow meter 02; the difference between the flow rates is yielded through an adder-subtracter 03; based on the difference, a PI control calculation means 04 outputs the control command signal θ; the outputted control command signal θ is converted into an EGR valve opening command signal, through an EGR valve opening command generator 05 (see FIG. 12); the signal θ is also converted into an intake air throttle valve opening command signal, through a throttle valve opening command generator 06 (see FIG. 12).
However, interlocking the movements of the EGR valve and the intake air throttle valve by a simple control command signal so as to operate both the valves as one as shown in FIGS. 11 and 12 contains some difficulties as described below.
Each function (the relation between the opening and the air/gas flow rate) of the valves has a dead zone (FIG. 11) in which the air/gas flow rate dose not change when the opening exceeds a certain level; accordingly, in the case where the intake air throttle valve is opening, the control command signal moves right from the point θx in FIG. 11, and the throttle valve is activated in an opening direction from the point P2 (FIG. 11) to the full opening point; thereby, the opening of the EGR valve moves right from the point P1 (FIG. 11), and continues to be fully opened in a dead zone Q (FIG. 11) for a while; thus, unless the EGR valve opening is narrowed to a certain degree, the EGR gas flow rate cannot decrease; in this way, even when the intake air throttle valve is activated toward the full opening, a sufficient amount of air cannot be quickly inhaled into the combustion chamber; as a result, there is a difficulty that the engine suffers from insufficient response performance.
Moreover, in the case where the EGR valve is opening, the control command signal moves left from the point θY in FIG. 11, and the EGR valve is activated in an opening direction from the point P2′ (FIG. 11) to the full opening point; thereby, the opening of the throttle valve moves left from the point P1′ (FIG. 11), and continues to be fully opened in a dead zone Q (FIG. 11) for a while; thus, unless the throttle valve opening is narrowed to a certain degree, the intake air flow rate cannot decrease; in this way, even when the EGR valve is activated toward the full opening, a sufficient amount of EGR gas (a sufficient EGR rate) cannot be quickly increased; as a result, there is a difficulty that the engine suffers from insufficient response performance.
Thus, in the case where the EGR valve movement is associated with the throttle valve movement as if the two valves were integrated into one valve, the response performance as to the engine acceleration as well as the EGR gas flow rate is prone to be deteriorated, due to the dead zone characteristics inherent to the EGR valve and the intake air throttle valve.